Polyphenol shelled nanoparticles lubricating composition and method

ABSTRACT

A compound for enhancing lubrication includes nanoparticles having a size less than 100 nm; and a polyphenol derived agent coating an external surface of the nanoparticles for enhancing the lubrication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/881,551, filed on Aug. 1, 2019, entitled “LUBRICATING COMPOSITION CONTAINING POLYPHENOL SHELLED NANOPARTICLES FOR ENHANCED LUBRICATION PERFORMANCES,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND Technical Field

Embodiments of the subject matter disclosed herein generally relate to nanoparticles (NPs) that are coated with gallic acid derivatives for enhancing the lubrication property of a product, and more particularly, to 2-octyl dodecyl gallic acid ester (ODG) that alters the surface properties of NPs for better dispersion stability in a synthetic based oil as polyalphaolefin (PAO) or fully formulated Helix 10W-30 engine oil (HEO).

Discussion of the Background

The automotive industry needs to improve the fuel economy of the produced cars. A major factor contributing to the loss of energy in a given car is the mechanical friction that develops between the various parts of the car. Advancing in lubricants can, on one hand, reduce the frictional energy loss, and, on the other hand, improve the mechanical operation efficiency of the car.

Minimizing the interfacial frictions in internal combustion engines by oil lubrication can be characterized into three lubrication regimes: boundary lubrication, mixed lubrication, and hydrodynamic (or film) lubrication. Reducing the friction at boundary lubrication and mixed lubrication regimes is particularly important. The friction induced by the surface asperity contacts between the various components of the engine at the boundary lubrication and mixed lubrication regimes are far greater than that in the hydrodynamic lubrication regime. Blending 0.5% to 1.0% nanoparticles (NPs) in the lubricating oil was shown to efficiently reduce the frictional energy loss as well as the severe wear tracks. Metal-based NPs, e.g., Fe, Cu, and Co, and metal oxide-based NPs, e.g., ZnO, CuO, ZrO₂ having sizes that range from 20 nm to 100 nm were concluded to be effective in reducing the friction in the boundary lubrication regime. The lubrication mechanism of adding NPs is believed to (1) polish the engine parts' surface asperities when rolling over the contact area, (2) roll over the surface like ball-bearings objects, and (3) supply additional cations in tribochemical reactions, which thicken the tribofilm on the surfaces. A tribofilm is considered herein the film or coating that forms at the surface of the NPs (the surface of other elements is also possible) due to various stresses.

Among all NPs applications in lubrication, TiO₂ NPs demonstrate good potential, not only because of its proven friction-modifying functionality, but also because of its availability, non-toxicity, inexpensiveness in the market, and easiness to be synthesized. Particularly, anatase phase TiO₂ was able to better reduce the Coefficient of Friction (COF) than rutile TiO₂ and mixed phases of TiO₂, albeit the wear behaviors were concerned. NPs could inhibit the main tribochemical reactions that form protective tribofilms on the contacting surface.

Moreover, the dispersion stability of hydrophilic NPs in non-polar synthetic oil, for instance, PAO, is of concern for the lubrication applications as the NPs tend to aggregate so that its surface energy can be reduced, which eventually results in quick sedimentation. Enhancing the dispersion stability of the NPs by using a dispersant or surfactant was also considered due to the weak electrostatic interactions. Since the affinity of the surface modifying agent could either be so strong that it collapses the structure of the NPs, or too weak to stably graft on the surface of the nanoparticles, proper surfaces modifying agents have been explored [1], such as carboxylic acids, polymer/copolymer, silanes, and organophosphorus while the effects of utilizing surface-modified NPs in lubrications and its related tribochemical mechanisms were recently discussed. Some of the existing surface modifying agents are now discussed.

JP 392593262 discloses methods to prepare the organically modified metal oxide nanoparticles. The organic modifiers, alcohols, aldehydes, carboxylic acids, amines, thiols, amides, ketones, oximes, phosgene, claim enamine, and amino acid are claimed to modify the surface of NPs. NPs size ranging from 5 nm to 100 nm or greater than 100 nm are discussed. Various kinds of metals, metal oxides, and inorganic materials are disclosed by this patent.

JP 2006143698A discloses a phenol dimer and its preparation method and application in enhancing the lubricating oil performances by improving viscosity, neutralizing acids derived from combustion or inhibiting oxidation during the engine combustion. The as-synthesized diphenol derived compounds were coupled with a metallic compound and serve as a detergent in the formulation of the engine oil. The over-based phenol dimer can further provide its multifunctionality as viscosity improver, antioxidant and neutralizer. The phenol dimer derivatives were not designed to enhance the nanoparticles dispersions. The as-disclosed phenol derived dimers were not involved with the metal oxides nanoparticles for enhanced lubrications.

CN 105247022A discloses a lubricant composition including a dispersant having a high molecular weight organ macromolecule modified metal nanoparticles. The lubricant composition has both a good stability and good anti-scaling properties. The metal nanoparticles are specified to be MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, ZrS₂, ZrSe₂, HfS₂, HfSe₂, PtS₂, ReS₂, ReSe₂, TiS₃, ZrS₃, ZrSe 3, HfS₃, HfSe₃, TiS₂, TaS₂, TaSe₂, NbS₂, NbSeP, and NbTe₂, preferably selected from the group of MoS₂, MoSe₂, WS₂, and WSe₂. The dispersant is specified to be polyamide with specific functional groups with molecular weight ranging from 1,000 to 10,000 Daltons. The specified dispersant enhances the nanoparticles dispersion stability in a base oil (a base oil includes hydrocarbons typically used in lubricants), where the amount of dispersant is ranging from 0.1 to 10 w.t %.

U.S. Pat. No. 9,150,813 B2 discloses a lubricating composition containing an aromatic compound. Certain aromatic compounds in the lubricating composition were disclosed to at least serve as anti-wear agent, friction modifier, extreme pressure agent, or lead, tin, or copper (typically lead) corrosion inhibitor.

U.S. Pat. No. 8,741,821 B2 discloses a method for reducing a friction coefficient of a lubricated surface, and a lubricant composition for reducing a friction coefficient between lubricated surfaces. The method includes providing an amount of metal-containing dispersed in a fully formulated lubricant composition containing a base oil of lubricating viscosity, wherein the nanoparticles have an average particles size ranging from about 1 to about 10 nanometers.

U.S. Pat. No. 7,994,105 B2 discloses a laser synthesis method to create dispersed nanoparticles. A combination of dispersed nano and microparticle treatment for engines enhances fuel efficiency and life duration and reduces exhaust emissions. In the patent disclosure, the nanoparticles are chosen from a class of hard materials, preferably alumina, silica, ceria, titanium, diamond, cubic boron nitride, and molybdenum oxide; the microparticles are chosen from a class of materials of layered structures, preferably graphite, hexagonal boron nitride, magnesium silicates and molybdenum disulphide.

However, the existing surface modifying agents fail to provide just the right affinity to the NPs surface, to avoid the collapse of the structure or the failure to graft to the surface of the NPs. Thus, there is a need for a surface modifying agent that avoids the problems noted above.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment, there is a compound for enhancing lubrication which includes nanoparticles having a size less than 100 nm, and a polyphenol derived agent coating an external surface of the nanoparticles for enhancing the lubrication.

According to another embodiment, there is an oil mixture with improved lubrication, and the oil mixture includes nanoparticles having a size less than 100 nm; a polyphenol derived agent coating an external surface of the nanoparticles for enhancing the lubrication; and an oil in which the nanoparticles coated with the polyphenol derived agent are distributed.

According to still another embodiment, there is a method for making a compound for enhancing lubrication, and the method includes providing nanoparticles having a size less than 100 nm; oxidizing an external surface of the nanoparticles with an oxidizing agent to initiate a functional group on the external surface; preparing a 2-octyl dodecyl gallic acid ester; placing the nanoparticles with the functional group into EtOH and water solution; and adding, in a dropwise manner, the 2-octyl dodecyl gallic acid ester into the EtOH and water solution to coat the nanoparticles with the 2-octyl dodecyl gallic acid ester.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a flow chart of a method for making a polyphenol derived agent;

FIG. 2 illustrates the chemical reactions taking place for forming the polyphenol derived agent;

FIG. 3 illustrates the chemical reactions taking place for coating nanoparticles with the polyphenol derived agent;

FIG. 4 is a flow chart of a method for coating nanoparticles with the polyphenol derived agent;

FIG. 5 illustrates the structure of the nanoparticles coated with the polyphenol derived agent;

FIGS. 6A to 6D illustrate specific chemical configurations of the polyphenol derived agent;

FIGS. 7A and 7B illustrate the infrared spectra of nanoparticles, the polyphenol derived agent, and the nanoparticles coated with the polyphenol derived agent;

FIGS. 8A and 8B illustrate the results of the thermogravimetric analysis of nanoparticles coated with different polyphenol derived agents;

FIG. 9 illustrates the dispersion stability of nanoparticles and nanoparticles coated with the polyphenol derived agent;

FIG. 10 illustrates the coefficient of friction and wear volume data of the coated TiO₂ nanoparticles in various oils;

FIG. 11 illustrates the coefficient of friction for the coated ZrO₂ nanoparticles in various oils;

FIGS. 12A to 12D illustrate the coefficient of friction for various oil mixtures including coated nanoparticles;

FIG. 13 illustrates the nanoindentation mechanical properties of the derived tribofilm in various oils;

FIG. 14 illustrates the chemical compositions of various oil mixes that include coated nanoparticles; and

FIG. 15 is a flowchart of a method for coating nanoparticles with a polyphenol derived agent.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to TiO₂ NPs. However, the embodiments to be discussed next are not limited to TiO₂ NPs, but may be applied to other NPs as discussed later.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

According to an embodiment, a novel method that modifies the surface of NPs using the building blocks of polyphenols is introduced and the novel structure obtained with this method is shown to enhance dispersions as well as lubrication performances of the material to which it is added. Further, the surface modified NPs, for example, inorganic based nanoparticles shelled by polyphenol compounds, show enhanced oil lubrication performances, including improved dispersion stability, wear volume reduction, friction reduction, and anti-oxidative stability. In this regard, the inventors have observed that the oxidation of hydroxyl groups on the phenol or polyphenol [2, 3] helps to improve the properties of the modified NPs. The mechanism of polyphenol surface chemistry has revealed that the hydroxyl group on the gallate ring can be easily deprotonated under mild conditions and the generated radicals can spontaneously bridge the polyphenol itself with the contact surface [4]. Such behaviors can be potentially applied for the efficient surface modification on NPs with great stability as well as enhanced lubrications for its suitable molecular structure with polarities and aromatic rings [5].

The inventors have further identified the differences in tribological behaviors as well as mechanisms between the ODG surface-modified TiO₂ NPs and unmodified TiO₂ NPs when mixing them with PAO 10 and HEO oils, respectively. The physical properties, including thermal properties, dispersion stability, and nanostructures, of the surface modified TiO₂ NPs were characterized before preparing these mixtures. With the aid of Optimol-SRV5 reciprocation tribometers in a steel ball-on-disk configuration, the coefficient of friction and the mean friction coefficient were determined. The friction and wear mechanisms were determined as discussed later.

According to an embodiment, anatase TiO₂ NPs with the single-particle size ranging from 20 to 25 nm were used. The surface modifying agent, ODG, was synthesized from the gallic acid monohydrate and 2-octyldodecanol with catalyst p-toluene sulfonic acid (PTSA). More specifically, similar to the procedures disclosed in [6] for preparing the ODG, gallic acid monohydrate was vacuum dried in step 100, as shown in FIG. 1, under 100° C. for 2 hours, to eliminate other side reactions due to the moisture. Gallic acid 210 (5.0 g, 30 mmol), 2-octyldodecanol 220 (26.8 g, 90 mmol), and PTSA 230 (0.05 g) were reacted (See FIG. 2) in the mixture of anisole and nitrobenzene (25 ml, molar ratio, 15:1) in step 102 under reflux condition at 160° C. for 12 hours, where the nitrogen was purged into the two-neck flask to prevent possible oxidations. After cooling in step 104, the prepared solution including a polyphenol agent 240 (i.e., the ODG) was dissolved in ethyl acetate (30 mL) in step 106 and washed successively in step 108 with saturated solutions of NaHCO₃ (100 mL) and NaCl (100 mL). The obtained organic layer made of the polyphenol agent 240 (ODG layer) was recrystallized in step 110 in petroleum ether and then dried in step 112 at 60° C. under vacuum, overnight. The residue was then purified by column chromatography using petroleum ether/EtOAc, (3:7) as the eluent.

The method [7] of coating TiO₂ NPs with the ODG 240, called herein ODG@TiO₂ NPs, was modified (see FIG. 4) so that the purified ODG 240 (2.245 g, 50 mmol) was drop-wise added in step 400 into an EtOH/water solution (50 ml, ratio=1:4, pH=4) containing TiO₂ NPs 250 (0.79 g, 10 mmol), as illustrated in FIG. 3. The TiO₂ solution is quickly turned from a white milky solution to orange colloidal in a few minutes. The acidic solution was applied for the surface modification on NPs as possible acid-catalyzed surface reactions could aid in the interactions between the surface of NPs 250 and hydroxyl groups on ODG 240. The mixed solution (TiO₂/ODG) was stirred in step 402 for 12 hours. A large amount of methanol (ca. 150 ml) was subsequently used in step 404 to remove the unreacted ODG. The obtained solution was centrifuged, and vacuum dried overnight, in step 406, to evaporate the methanol. The remaining orange/dark red fine power 260 that includes TiO₂ NPs 250 coated with the ODG layer 240, called herein ODG@TiO₂, was collected to be mixed with PAO and/or HEO.

The novel ODG@TiO₂ material 260, which includes the TiO₂ NPs 250 coated with the ODG layer 240, is show in FIG. 5 and this new material (composition) has been found to provide at least one of anti-wear performance, friction modification for enhancing fuel economy, extreme pressure performance, and antioxidant performance. FIG. 5 further shows the novel ODG@TiO₂ material 260 placed inside an oil 510, which may be PAO, HEX, or any other commercially available oil. The ODG@TiO₂ material 260 mixed with the oil 510 makes up an oil mixture 500.

In one embodiment, the dispersion stability of the polyphenols shelled NPs 260 may be enhanced through the intermolecular interactions among the (1) polyphenol shelled layer, (2) the non-polar base oil (e.g., olefins with more than 20 carbons and viscosity ranging from 80 to 200), and (3) other additives, such as detergents, dispersants, and viscosity modifiers, which are discussed later.

In this or another embodiment, the polyphenol shelled NPs 260 act as a lubrication additive, and the NPs 250 may be organics, inorganics, metal or metal oxides that can be shelled by the building block of polyphenol derived compounds. The materials of NPs are not limited to organics, inorganics, metal or metal oxides as long as the surface of NPs can be modified and capable of being shelled by polyphenols (e.g., NPs with a single element are Fe, Cu, Zn, Ag, Pd, Ni, or Co; or with metal oxides, such as ZnO, TiO₂, ZrO₂, FeO, Fe₂O₃, or CeO₂, or organics, such as polymers, graphene or carbon nanotube).

In this or another embodiment, the building blocks of the polyphenol shell 240 that modify the surface of the NPs 250 are described by the formulae shown in FIGS. 6A to 6D. The formulae shown in FIGS. 6A to 6D generally contain a ring 600 with at least one polar part Y¹ 610 and at least one long alkyl R¹ chain 620 to ensure the applicability of the compounds modifying the surface of NPs 250, dispersion stability enhancement in pure or modified base oils, and lubrication performances improvements. This novel approach is further designed and engineered to bind the surface of the NPs 250 with the phenol derived compounds 240 shown in the formulae of FIGS. 6A to 6D instead of directly blending the aromatic compounds in the lubricating compositions as previously performed in the art.

The various elements shown in FIGS. 6A to 6D are as follow: R¹ may be a linear or branched hydrocarbyl group containing 1 to 350 carbon atoms. In one application, the R¹ may be replaces by —C(O)XR¹, or —CH═CHC(O)XR³, or —C(R⁴)₂C(R⁴)₂C(O)XR³ (such as —CH₂CH₂C(O)XR³), (typically R¹ may be a hydrocarbyl group derived from a polyalkene, or —C(O)XR³). Each Y¹, Y², and Y³ may be independently —OH, —COOH, —COH, or —OR² (typically Y¹, Y², and Y³ are at least containing —OH, —COOH, or —CON); R² may be independently hydrogen or a linear or branched hydrocarbyl group containing 1 to 20 carbon atoms; R³ may be a linear or branched hydrocarbyl group (typically alkyl, aryl, alkaryl, alkoxy, aryloxy) that contains 1 to 40, 3 to 30, 4 to 30, 5 to 30, 6 to 30, 8 to 24, 8 to 20, 8 to 18, 5 to 10, or 10 to 18 carbon atoms; R⁴ may be hydrogen or a linear or branched hydrocarbyl group containing 1 to 5, or 1 to 2 carbon atoms, (typically R⁴ is hydrogen); and X may be —O—, —S—, or —NR², but typically X may be —O—.

A few specific examples of the new material 260 are now discussed with the purpose of teaching how to make the invention, and not to limit the invention to these specific compositions. The formulae shown in FIGS. 6A to 6D may chemically or physically modify the surface properties of the NPs 250 through proper chemical reactions in proper solvents at suitable temperatures. In one example, the shelling polyphenol compounds modified the surface of the NPs 250 as discussed next. The responsible compound is a chemically modified gallic acid long chain ester, 2-octyldodecyl gallate (ODG), but the application of the NP shelling or coating agent is not limited to the compounds of gallic acid ester.

In one example of shelling polyphenols, proper modification on the surface of the NPs 250 before shelling may be required. The method may involve a strong oxidation agent, such as sulfuric acid, potassium permanganate, hypochlorite, hydrogen peroxide, or sodium hydroxides to grow —OH groups on the surface of the NPs 250. TiO₂ and ZrO₂ are commercially available and having diameters ranging from 10 nm to 100 nm. In this embodiment, an amount of 0.05 g TiO₂ or ZrO₂ NPs is treated in 50 ml, 0.05M sulfuric acid and stirred for 6 hours to initiate the —OH functional groups on the surface of the NPs. The —OH functional groups may interact with the polyphenol derived compounds 240 so that the NPs 250 can be shelled with polyphenols derived compounds to form the final material 260.

Regarding the polyphenol shell layer 240 shown in FIG. 5, the composition described by the formula shown in FIG. 6C may be synthesized as discussed with regard to FIG. 1. The surface engineered NPs 250 may then be shelled with a layer or with multilayers of polyphenols 240 as discussed with regard to FIG. 4. The synthesized polyphenol shelled NPs 260 shown in FIG. 5 were confirmed to be successful when characterized by the Fourier transform infrared (FTIR), see FIGS. 7A and 7B, and by thermogravimetric analysis (TGA), see FIGS. 8A and 8B. FIG. 7A shows the spectra of the ODG 240, TiO₂ NPs 250, and the ODG@TiO₂ 260. It is noted that the transmittance of the ODG@TiO₂ 260 becomes about 60% for a wavenumber between 1500 and 1000.

The successful surface modification of the NPs was confirmed by IR spectroscopy as shown in FIG. 7A. The hydroxyl group signal (between 3450 cm⁻¹ and 3350 cm⁻¹) of ODG disappeared after covering the surface of TiO₂ with the ODG. The ODG and the NPs were linked together in bidentate mononuclear or binuclear structure through the hydroxyl groups according to the scheme illustrated in FIG. 3. The bands observed at 2953 cm⁻¹ and 2869 cm⁻¹ were due to the C—H stretching of the ODG. The C—O stretching at 1320 to 1210 cm⁻¹ in FIG. 7A resulted from the carbonyl functionality of the ester. The broad signals vibrations from 1400 to 1600 cm⁻¹ were because of the aromatic ring stretching. Being provided with the evidence of the obvious signals at 738 cm⁻¹ characterized by the TiO₂ NPs as well as the alkyl chain, the IR spectra confirmed the successful surface modification of the TiO₂ NPs grafted by ODG. FIG. 7B shows the spectra of ODG 240, ZrO₂ NPs 252, and the ODG@ZrO₂ 260.

FIG. 8A shows the mass change when the ODG@TiO₂ 260 is heated at a rate of 10° C./min with the N2 purged at 20 cm³/min and the change in mass is about 3% over a large temperature range, while FIG. 8B shows the mass change when the ODG@ZrO₂ 262 is heated as in FIG. 8A, and the change in mass is less than 1%.

The surface properties of the synthesized ODG@TiO₂ 260 and ODG@ZrO₂ 262 can be altered from hydrophilic into hydrophobic. For instance, the dispersion stability of the polyphenol modified nanoparticles, i.e., TiO₂ shelled by ODG (ODG@TiO₂) 260 shows enhanced dispersion stability in hexadecane, as illustrated in FIG. 9, when compared with the traditional TiO₂ NP 250 uncoated by the ODG 240. In this respect, 50 mg of prepared nanoparticles TiO₂ for the first experiment and ODG TiO₂ NPs for the second experiment, were dispersed in 50 ml of hexadecane and sonicated for 10 minutes before the UV-vis spectroscopy scanning. The concentration of the suspended NPs was detected through the UV-vis light absorbance signal according the Beer-Lambert Law, as shown in FIG. 9.

The dispersing stability of the NPs was improved overall after the surface modification with the ODG. The non-modified TiO₂ quickly settles down in hexadecane during the first 10 minutes while the ODG@TiO₂ started to settle down after approximately 60 minutes. The hexadecane was used in this study because the exact structure and full functionality of the PAO cannot be fully characterized. Using hexadecane as the standard can help to evaluate the degree of the aliphatic intermolecular interactions as well as the improved dispersion stability when TiO₂ NPs surfaces were being modified by ODG.

The dispersion stability for the ODG@ZrO₂ 262 and the ODG@TiO₂ 260 has been estimated to be up to 180 days, depending on the dimension of the NPs and the polyphenol intermolecular interactions with functionalities carried by the base oils and other additives, such as dispersants, detergents, and viscosity modifiers. In this regard, it is noted that when blending the shelled NPs with other dispersants, detergents, and viscosity modifiers in full mixtures, as discussed later, and when controlling the NPs size range to be from 1 to 20 nm, the dispersion stability of the polyphenol shelled NPs 260/262 can be from 1 to 3 years. The polyphenol shelled NPs 260/262 can also be used as an antioxidant to improve the thermal/oxidative stability of the oil.

In the examples of the ODG shelled TiO₂ and ZrO₂ NPs discussed above, the lubrication performances of a base oil, either HEO or PAO10, was enhanced after adding the ODG@TiO₂ or ODG@ZrO₂, when compared to those of non-shelled ones as well as the pure base oils. With the aid of a reciprocation tribometers friction tester with a steel ball-on-disk configuration, the coefficient of friction (COF), mean friction coefficient, and wear volume were determined and they are presented in Tables 1 and 2 in FIGS. 10 and 11, respectively.

The wear volumes were characterized using an Optical Profiler. FIGS. 12A to 12D show that the COF for both the ODG@TiO₂ and ODG@ZrO₂ mixtures is reduced when added to commercial Helix 10W-30 engine oil (HEO) as well as the polyalphaolefin 10 (PAO10) oil. In this regard, FIGS. 12A to 12D illustrated the COF for various mixtures of oil and the novel material 260 over a given time interval. In this regard, the polyphenol shelled NPs 262, 1.0% ODG@ZrO₂ in HEO, demonstrates a better compatibility with the fully formulated commercial engine oil. Because friction reductions are negatively correlated with the wear volumes, it is expected that the wear volumes in the ODG@ZrO₂ are less than those in ODG@TiO₂. The polyphenol shelled NPs 260, 1.0% ODG@TiO₂ in HEO, show continuous friction reductions when the time increases. Thus, by mixing the polyphenol shelled NPs 260 with an existing oil, a synergistic effect of friction reduction and better fuel economy may be obtained.

FIGS. 12A to 12D show a better tribological performances of the 1.0% ODG@TiO₂ NPs than that of TiO₂ NPs. In HEO, the 1.0% ODG@TiO₂ blend can reduce the mean COF by 7.4% while the 1.0% TiO₂ blend can only decrease the mean COF by 2.9% when setting HEO as the benchmark. Furthermore, the mean COF of 1.0% ODG@TiO₂ NPs in PAO10 was significantly lower by 18.5% as compared with that of pure base oil PAO. Surprisingly, the mean COF of 1.0% TiO₂ blend in PAO10 was shown to increase by 3.1% as compared with the base oil PAO 10. This finding of increased friction achieved by the addition of 1.0% TiO₂ could emphasize the importance of surface properties of NPs for better dispersions as well as the lubrication performances.

Besides, the wear volume was increased by the addition of TiO₂ NPs in both PAO and HEO, which could be mainly due to the sintered NPs on the contacting surface as observed from the surface morphologies on the wear tracks. The wear volume and mean COF was reduced by 2.1% and 7.4% respectively when formulating 1.0% ODG@TiO₂ in HEO. The tribological behavior discrepancies between HEO and PAO can be due either to the zinc dialkyldithiophosphates (ZDDPs) derived tribofilm, or the elimination of NPs aggregations that ensure the dynamic ball-bearing rolling effects.

The NPs sedimentation kinetics can be evaluated through various methods, while applying UV-vis spectroscopy was found to be more reliable. The relative concentration of the suspended NPs in the base oil is related to the light transmission intensity through the colloidal, according to the Beer-Lambert Law:

A=ε·c·l,

where A is the absorbance, c is the molar extinction coefficient (M⁻¹ m⁻¹), c is the concentration (mol L⁻¹), and I is the path length (m). As the chemical structure of the PAO could vary depending of its provider, a surrogate for the PAO was chosen, e.g., hexadecane. For these measurements, 50 mg of each of the NPs, TiO₂ and the prepared ODG@TiO₂ were dispersed in 50 ml hexadecane respectively and sonicated for 10 minutes before the UV-vis spectroscopy scanning.

To evaluate the tribological performance of the ODG@TiO₂ 260 in PAO and HEO, which is illustrated in FIGS. 10 and 11, the NPs blends were prepared at 0.5 wt % and 1.0 wt %. The 1.0% TiO₂ NPs blending ratio was set as the benchmark for the comparisons with ODG@TiO₂, considering the best blending concentration for enhanced tribological performances using TiO₂. All testing balls and disks were cleaned with petroleum ether and acetone in sonication bath and dried before the tests to prevent contaminations. The measurements of friction coefficients for each formulated sample were conducted by using the following conditions: load 100N at 25 Hz oscillation with a 1 mm stroke for 30 minutes under 50° C. in SRV5 ball-on-flat setup. In the ball-on-flat tribotest, polished DIN 51834 10 mm steel balls were reciprocated on the DIN 51834 24×7.9 mm steel. After the friction evaluations, the surface wear volumes of the tested disk and ball were quantized using an optical profiler.

The surface elemental mapping of wear scars was done using SEM coupled with EDS. The wear scar on the disk was lifted 10 mm by gallium ion source using FIB, where the thickness of the tribofilm was quantized and imaged with the qualitative mapping on elemental compositions using the EDS. Before the cross-sectional analysis on the wear scars using the FIB, the tribofilm was covered with protective platinum (ca 0.5 μm) using microscopy pen followed by electron beam deposition to form a protective platinum layer.

The nanoindentation mechanical properties of the derived tribofilms were determined using the Nanotest Vantage with a standard diamond Berkovich tip. The calibration of the standard diamond Berkovich tip was performed before the measurements, where the calibration curves were used to construct the function for fitting the data and determining the hardness and reduced modulus of the contacting substrates. 25 sampling indentation points were set with an indentation depth from 10 nm to 200 nm. The initial loading of the diamond Berkovich tip on the sample was set to be 0.03 mN with loading and unloading rate at 1 mN/s. After the measurements, suitable unloading curves within the tribofilm were collected and calculated for determining the hardness and reduced modulus and the results are illustrated in FIGS. 10 and 11.

As characterized by the TEM-EDS-FIB, the tribofilm elemental composition profile in HEO formulations is composed mainly of P, S, and Zn, which are the compositional profile derived from the ZDDP additive. The tribofilm derived from the TiO₂/HEO mixture was found to be mainly made up of the Zn/S/P-rich with a minor amount of Ti filling into the film. The tribofilm derived from the ODG@TiO₂ material demonstrated a much greater amount of Ti filling in the ZDDP derived tribofilm. These tribofilm elemental composition differences were attributed to the added ODG@TiO₂ or TiO₂ in HEO.

The thickness of the tribofilm derived from 1.0% TiO₂/HEO (c.a. 10 nm) was measured to be thinner than that of 1.0% ODG@TiO₂ in HEO (c.a. 50 nm-75 nm) and pure HEO (c.a. 50 nm). The additives in the pure HEO and 1.0% ODG@TiO₂/HEO might be able to be adsorbed by the contacting surface and form a protective layer to prevent the direct asperity contacts that generate wears. These behaviors were attributed to the effective ZDDP in lubrications as well as the specific tribochemical reactions that aid in the formation of protective tribofilm. On the other hand, the 200 nm-scale cross-sectional view on tribofilm generated in the TiO₂/HEO formulation was found to be relatively rough, which could suggest the possible mechanical scuffing processes as done by the micro ball-bearing rolling of TiO₂ NPs or the sintered TiO₂ NPs that creates the contact asperities.

The tribofilm derived from the PAO based mixtures was found to be different in the HEO based mixtures. The cross-sectional tribofilm analysis on the contacting surface of the TiO₂/PAO mixture was found to be discontinuous across the contacting surface line. The TiO₂ NPs seemed to be simply sintering into the contacting steel surface instead of forming the complete tribofilm. Two aggregation sites of the TiO₂ were observed. The sintered NPs could directly become as the contacting asperities or serve as a sintering core for more aggregations in the following reciprocations. Meanwhile, the derived tribofilm in the ODG@TiO₂ NPs in PAO was shown to form complete tribofilm over the contacting surface and on average thicker than that derived from the TiO₂/PAO. This might explain why it is observed the tribofilm formation from the ODG@TiO₂ instead of TiO₂ in PAO, considering the enhanced intermolecular interactions achieved by the ODG.

The nanoindentation experiments performed on the derived tribofilm of each mixture were carried out to associate the anti-wear performances and possible mechanisms from tribomechanical point of view, as illustrated in Table 3 in FIG. 13. The pure PAO oil gave similar mechanical properties as given by the stainless steel, which might be due to the little tribofilm formed on the contacting surfaces. For other PAO mixtures, great variances were found in both the hardness and reduced modulus, which may be associated with the cross-sectional results of the inhomogeneous surfaces. The reductions in hardness, as well as reduced modulus of the TiO₂ and ODG@TiO₂ blends in PAO, may be associated with the organic compounds filling into the TiO₂ derived film. On the other hand, a smaller variance of nanoindentation mechanical properties was measured in the ODG@TiO₂ and pure HEO formulations, but not in the TiO₂/HEO mixture. Similar to the reasons of the variances found in the PAO mixtures, the results were dependent on the degree of homogeneity of the indented surfaces.

The nanoindentation results of the ZDDP derived tribofilm were found to be ranging from 1 to 9 GPa for the hardness and approximately 100 GPa for the reduced modulus. The resulted variances of the ZDDP derived tribofilm were attributed to the indentation depth, the contacting parameters, and the presence of other additives in the formulations. The anti-wear behavior can be demonstrated by either the high hardness of the tribofilm so that the frictional energies were able to be dissipated by the elastic asperity contacts, or the tribofilm with the low reduced modulus so that the plastic film can flow between two contacting surfaces. The nanoindentation mechanical properties in the HEO mixtures demonstrate a trend of increasing hardness as well as the increasing reduced modulus when incorporating a larger amount of Ti element into the ZDDP tribofilm. The incorporation of the ODG and TiO₂ in the ZDDP derived tribofilms could strengthen the hardness as well as increase the reduced modulus of the derived films. As a greater hardness and reduced modulus were measured in the ODG@TiO₂/HEO, a possible explanation to the improved anti-wear performances could be the results of the stiffened tribofilms, which facilitated the dissipation of the frictional energy during the reciprocation.

Overall, the characterized tribological behaviors and properties of the PAO or HEO mixtures using the ODG modified TiO₂ NPs 260 and unmodified ones can be attributed to the following mechanisms:

The surface modifying agent ODG was able to enhance the adsorption of the ODG@TiO₂ NPs and create a stronger binding with the contacting surface as compared to the unmodified TiO₂ NPs;

The enhanced intermolecular interactions (adhesive effects) could be due to the pi-pi interactions by the aromatic rings; and

The micro ball-bearing TiO₂ NPs eliminate the asperity contacts as well as separate two contacting surfaces from direct contact while the aggregated NPs hinder its dynamics in ball-bearing rolling.

The morphology analysis using the SEM on both mixtures, PAO and HEO having TiO₂ NPs, suggested the role of the TiO₂ in the wear mechanism. The surface morphology in the TiO₂ NPs mixtures was scuffed after the sliding and clearly with the observation of sintered TiO₂ NPs on the contacting surface, regardless of the types of base oil. Besides, the surface roughness of the pure TiO₂ formulations in either the PAO or HEO was observed to be significantly greater than that of the ODG@TiO₂ based on the SEM images. As the correlation between the surface roughness and the amount of large agglomerates was further analyzed from the SEM images, the wear volume seemed to positively correlate to the number of agglomerates (or roughness), where the wear volume was ranked in the following order: 1.0% ODG@TiO₂/PAO >1.0% TiO₂/PAO >1.0% TiO₂/HEO >1.0% ODG@TiO₂/HEO. On the other hand, the reduction in friction can be summarized in the following order: 1.0% ODG@TiO₂/HEO >1.0% TiO₂/HEO >1.0% TiO₂/PAO >ODG@TiO₂/PAO. The improved tribological behaviors can be generally related to the surface modified NPs, mechanical properties of the tribofilm, and the amount of TiO₂ NPs incorporating into the ZDDP derived tribofilms.

Under high contacting pressure, the TiO₂ NPs can become stiff and become sintered into the contacting surface, and subsequently form the contacting asperities by aggregation. High-frequency reciprocation could result in the removal of the sintered NPs along with the brittle cast iron. Smoother agglomerates on the surface were observed only in the ODG@TiO₂ material when placed in the HEO, but not when placed in the PAO. This might be the result of over-pronounced adhesive effects produced by the organic shell layer since no other kinds of additives were present in the PAO. Besides, the presence of the ZDDP could explain why the smoother agglomerates and less severe wear track were observed when adding the ODG@TiO₂ to the HEO rather than in the PAO. The ZDDP in the HEO may balance the ODG adhesive effects under high pressures and synergize with the typical metalorganic tribofilm derived from the ZDDP and even thicken the generated tribofilm.

The TEM-EDS-FIB results suggest that the tribochemical interactions between the ZDDP and ODG@TiO₂ happen on the contacting surface rather than in the bulk lubricant. Although the competition in terms of the adsorption between the ODG@TiO₂ and ZDDP was not conclusive, the ODG@TiO₂ demonstrated the greater amount of adsorption on the contacting surface as compared to the pure TiO₂ in either the PAO or HEO.

As soon as other additives were added to the base oil, synergistic effects were achieved by the mixture, which can be explained by the formation of a thicker tribofilm. The mixtures may include ionic liquids, MoDTC, and some dispersants. The TEM-EDS-FIB cross-sectional analysis on the wear track of the HEO mixtures including the ODG@TiO₂ and TiO₂ might be associated with synergistic effects similar to the anti-wear and anti-friction mechanisms. The synergistic effect in the material 260 is due to the tribofilm derived from the ODG@TiO₂ in the HEO mixture, which was found to have enriched the Ti element filling into the ZDDP derived tribofilm as well as a thickened tribofilm, while a similar behavior was not found in either the TiO₂/HEO mixture or the pure HEO.

As the TiO₂ material is considered to be a chemically-hard species, which might destroy the chemically-soft ZDDP derived tribofilm, while the surface-modified TiO₂ having the coat of ODG is softer as the ODG layer softens the chemically hard-TiO₂, it is possible that the ODG coating prevents the tribomechanical scuffing of the derived tribofilms. Therefore, the enhanced tribological properties were generally better for the ODG@TiO₂ material than for the oil mixed only with the TiO₂ NPs. Thus, it was found that the surface modifying agent, ODG, could not only enhance the NPs dispersion stability in non-polar lubricant base oil, but also facilitate the tribofilm growth, and even generate a tribofilm with desired mechanical properties by incorporating the ODG modified NPs into the ZDDP derived tribofilm.

Therefore, based on the novel work disclosed herein, the synthesized core-shell ODG@TiO₂ NPs 260 was shown to have better dispersion stability as demonstrated by the sedimentation kinetics using UV-vis spectroscopy combined with the application of the Beer-Lambert's law. The improved dispersion stability in the formulated lubricant is attributed to the intermolecular interactions between the designed surface modifying agent, ODG, on the TiO₂ NPs and the aliphatic part in the polyalphaolefin to suspend the NPs in the base oils. Regarding the tribological properties, obtaining 1.0% ODG@TiO₂ in PAO can improve the mean COF reduction by up to 18.5% as compared with the pure PAO, but shows no improvements in anti-wear performances. On the other hand, ODG@TiO₂ NPs were able to reduce both the friction and wear volume when blending with the HEO. As characterized by TEM-EDS-FIB, the pure TiO₂ NPs were not able to provide the tribological advantages in either HEO or PAO blends.

In the ODG@TiO₂ modified oils, thicker tribofilm formation on the contacting steel was observed in both the HEO and PAO, due to the intermolecular interactions provided by the ODG. The best results were observed when formulating 1.0% ODG@TiO₂ with the HEO that contains the ZDDPs. The undesirable wear behavior when blending the ODG@TiO₂ in the PAO could be the result of over-pronounced intermolecular interactions since the sintered TiO₂ NPs and ball-bearing rolling NPs were adhesive during the high-frequency reciprocating and no other additives were present in the PAO to eliminate the over-pronounced adhesive effects. Thus, it is believed that the polyphenol derived surface modifying agent could enhance the lubrication as well as the dispersion stabilities in the commercial lubricants.

In one application, the polyphenol shelled NPs 260 is incorporated into an existing lubricant in a range of 0.01 wt % to 10 wt % of the lubricating composition. The lubricating composition containing the polyphenol shelled NPs 260 can be formulated with different concentrations and elements as shown in Table 4 in FIG. 14. Table 4 exemplifies three different embodiments A to C, where the weight percentage of the ODG NPs 260 varies.

The dispersant noted in Table 4 may be an additive for preventing formation of organic deposits on the machine elements. For example, such a dispersant may be any of the known hydrocarbon succinimides. A dispersant viscosity modifier, e.g., acrylate polymer, is an additive that can serve as dispersant as well as improve the lubricant viscosity. An over-based detergent, e.g., phenolates, sulfonates, can be added to scavenge acidic contaminants from the lubricant. An antioxidant, alkyl sulfides, hindered phenols, is an additive that increases the lifetime of the lubricants. An anti-wear agent, e.g., esters, chlorinated paraffins, ZDDP, is an additive that protects the surface by producing an anti-wear tribofilm. A friction modifier, e.g., boron nitride, graphite, is an additive that reduces the friction by forming a thin organic layer on the surface. A viscosity modifier, e.g., co-polymers of polyalkyl methacrylates, is an additive that improves the viscosity index of the lubricant. Other performance additives, e.g., dimethyl silicones (anti-foaming), organic acids, alkaline compounds (rust and corrosion inhibitors) are added according to specific environments or configurations as needed. While only the HEX and PAO oils have been discussed herein, other type of oils (e.g., any base oil) may be used with the additives shown in FIG. 14.

A method for making the compound 260 for enhancing lubrication is now discussed with regard to FIG. 15. The method includes a step 1500 of providing nanoparticles 250 having a size less than 100 nm, a step 1502 of oxidizing an external surface of the nanoparticles 250 with an oxidizing agent to initiate a functional group on the external surface, a step 1504 of preparing the 2-octyl dodecyl gallic acid ester 240 as discussed above with regard to FIG. 1, a step 1506 of placing the nanoparticles 250 with the functional group into EtOH and water solution, and a step 1508 of adding in a dropwise manner the 2-octyl dodecyl gallic acid ester 240 into the EtOH and water solution to coat the nanoparticles 250 with the 2-octyl dodecyl gallic acid ester 240, as discussed with regard to FIG. 4. The oxidizing agent may be one or more of sulfuric acid, potassium permanganate, hypochlorite, hydrogen peroxide, or sodium hydroxides.

The compound 260 obtained by using the above method includes nanoparticles 250 having a size less than 100 nm, and a polyphenol derived agent 240 coating an external surface of the nanoparticles 250 for enhancing the lubrication. In one application, the nanoparticles are made of TiO₂. In this application, the TiO₂ nanoparticles are in anatase phase and have a size between 20 and 25 nm. In another application, the nanoparticles are made of ZrO₂. The polyphenol derived agent is a gallic acid ester. In one application, the gallic acid ester is a 2-octyl dodecyl gallic acid ester (ODG). In one embodiment, the polyphenol derived agent includes a polar functional group. The functional group includes one or more of (B—COOH)—R, (B—COH)—R, (B(OH)n)-R, or (O═B—O)—R, where B is a benzyl ring, R is an alkyl chain, C is carbon, O is oxygen, and H is hydrogen. In another application, the polyphenol derived agent includes a ring (600) having (i) a polar part Y¹ (610) and (ii) an alkyl chain R¹ (620). The alkyl chain R¹ is a linear or branched hydrocarbyl group containing 1 to 350 carbon atoms. The alkyl chain R¹ may be replaced by —C(O)XR¹, or —CH═CHC(O)XR³, or —C(R⁴)₂C(R⁴)₂C(O)XR³. The polar part Y¹ is —OH, —COOH, —COH, or —OR², R² is hydrogen or a linear or branched hydrocarbyl group containing 1 to 20 carbon atoms, R³ is a linear or branched hydrocarbyl group having 1 to 40 carbon atoms, R⁴ is hydrogen or a linear or branched hydrocarbyl group containing 1 to 5 carbon atoms, and X is —O—, —S—, or —NR².

The disclosed embodiments provide a special coating to nanoparticles and these treated nanoparticles are distributed into an oil for increasing the lubrication of the oil. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

REFERENCES

-   [1] S. Mallakpour and M. Madani, Prog. Org. Coatings, 2015, 86,     194-207. -   [2] T. S. Sileika, D. G. Barrett, R. Zhang, K. H. A. Lau and P. B.     Messersmith, Angew. Chemie, 2013, 125, 10966-10970. -   [3] A. Arbenz and L. Avérous, Green Chem., 2015, 17, 2626-2646. -   [4] W. C. Ault, J. K. Weil, G. C. Nutting and J. C. Cowan, J. Am.     Chem. Soc., 1947, 69, 2003-2005. -   [5] I. Minami and Ichiro, Appl. Sci., 2017, 7, 445. -   [6] W. C. Ault, J. K. Weil, G. C. Nutting and J. C. Cowan, J. Am.     Chem. Soc., 1947, 69, 2003-2005. -   [7] I. A. Jankovic, Z. V Saponjic, E. S. Dźunuzovic and J. M.     Nedeljkovic, Nanoscale Res Lett, 2010, 5, 81-88. 

1. A compound for enhancing lubrication, the compound comprising: nanoparticles having a size less than 100 nm; and a polyphenol derived agent coating an external surface of the nanoparticles for enhancing the lubrication.
 2. The compound of claim 1, wherein the nanoparticles are made of TiO₂.
 3. The compound of claim 2, wherein the TiO₂ nanoparticles are in anatase phase and have a size between 20 and 25 nm.
 4. The compound of claim 1, wherein the nanoparticles are made of ZrO₂.
 5. The compound of claim 1, wherein the polyphenol derived agent is a gallic acid ester.
 6. The compound of claim 5, wherein the gallic acid ester is a 2-octyl dodecyl gallic acid ester (ODG).
 7. The compound of claim 1, wherein the polyphenol derived agent includes a polar functional group.
 8. The compound of claim 7, wherein the functional group includes one or more of (B—COOH)—R, (B—COH)—R, (B(OH)n)-R, or (O═B—O)—R, where B is a benzyl ring, R is an alkyl chain, C is carbon, O is oxygen, and H is hydrogen.
 9. The compound of claim 1, wherein the polyphenol derived agent includes a ring having (i) a polar part Y¹and (ii) an alkyl chain R¹.
 10. The compound of claim 9, wherein the alkyl chain R¹ is a linear or branched hydrocarbyl group containing 1 to 350 carbon atoms.
 11. An oil mixture with improved lubrication, the oil mixture comprising: nanoparticles having a size less than 100 nm; a polyphenol derived agent coating an external surface of the nanoparticles for enhancing the lubrication; and an oil in which the nanoparticles coated with the polyphenol derived agent are distributed.
 12. The oil mixture of claim 11, wherein the nanoparticles are made of TiO₂ in anatase phase and have a size between 20 and 25 nm.
 13. The oil mixture of claim 11, wherein the nanoparticles are made of ZrO₂.
 14. The oil mixture of claim 11, wherein the polyphenol derived agent is a gallic acid ester.
 15. The oil mixture of claim 14, wherein the gallic acid ester is a 2-octyl dodecyl gallic acid ester (ODG).
 16. The oil mixture of claim 11, wherein the polyphenol derived agent includes a polar functional group.
 17. The oil mixture of claim 16, wherein the functional group includes one or more of (B—COOH)—R, (B—COH)—R, (B(OH)n)-R, or (O═B—O)—R, where B is a benzyl ring, R is an alkyl chain, C is carbon, O is oxygen, and H is hydrogen.
 18. The oil mixture of claim 11, wherein the polyphenol derived agent includes a ring having (i) a polar part Y¹ and (ii) an alkyl chain R¹.
 19. The oil mixture of claim 18, wherein the alkyl chain R¹ is a linear or branched hydrocarbyl group containing 1 to 350 carbon atoms.
 20. A method for making a compound for enhancing lubrication, the method comprising: providing nanoparticles having a size less than 100 nm; oxidizing an external surface of the nanoparticles with an oxidizing agent to initiate a functional group on the external surface; preparing a 2-octyl dodecyl gallic acid ester; placing the nanoparticles with the functional group into EtOH and water solution; and adding, in a dropwise manner, the 2-octyl dodecyl gallic acid ester into the EtOH and water solution to coat the nanoparticles with the 2-octyl dodecyl gallic acid ester. 